Methods of fabricating a three-dimensional non-volatile memory device

ABSTRACT

A method of fabricating a semiconductor device, such as a three-dimensional NAND memory string, includes forming a first stack of alternating layers of a first material and a second material different from the first material over a substrate, removing a portion of the first stack to form a first trench, filling the trench with a sacrificial material, forming a second stack of alternating layers of the first material and the second material over the first stack and the sacrificial material, removing a portion of the second stack to the sacrificial material to form a second trench, and removing the sacrificial material to form a continuous trench through the first stack and the second stack.

RELATED APPLICATION

This application claims the benefit of priority to U.S. ProvisionalApplication No. 61/879,074, filed on Sep. 17, 2013, the entire contentsof which are incorporated herein by reference.

FIELD

The present invention relates generally to the field of semiconductordevices and specifically to three dimensional non-volatile memory, suchas vertical NAND strings, and other three dimensional devices andmethods of making thereof.

BACKGROUND

Recently, ultra high density storage devices have been proposed using athree-dimensional (3D) stacked memory structure sometimes referred to asa Bit Cost Scalable (BiCS) architecture. For example, a 3D NAND stackedmemory device can be formed from an array of alternating conductive anddielectric layers. A memory hole is formed through the layers to definemany memory layers simultaneously. A NAND string is then formed byfilling the memory hole with appropriate materials. A straight NANDstring extends in one memory hole, while a pipe- or U-shaped NAND string(p-BiCS) includes a pair of vertical columns of memory cells. Controlgates of the memory cells may be provided by the conductive layers.

SUMMARY

Embodiments relate to a method of fabricating a memory device thatincludes forming a first stack of alternating layers of a first materialand a second material different from the first material over asubstrate, removing a portion of the first stack to form a first trench,filling the trench with a sacrificial material, forming a second stackof alternating layers of the first material and the second material overthe first stack and the sacrificial material, removing a portion of thesecond stack to the sacrificial material to form a second trench, andremoving the sacrificial material to form a continuous trench throughthe first stack and the second stack.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A-1B are respectively side cross sectional and top crosssectional views of a NAND string of one embodiment. FIG. 1A is a sidecross sectional view of the device along line Y-Y′ in FIG. 1B, whileFIG. 1B is a side cross sectional view of the device along line X-X′ inFIG. 1A.

FIGS. 1C-1D are respectively side cross sectional and top crosssectional views of a NAND string of another embodiment. FIG. 1C is aside cross sectional view of the device along line Y-Y′ in FIG. 1D,while FIG. 1D is a side cross sectional view of the device along lineX-X′ in FIG. 1C.

FIG. 2 is a partial side cross-sectional view of a memory devicecomprising a plurality of NAND strings formed in a stack of materiallayers over a substrate.

FIG. 3A is a top cross sectional view of a memory block comprising aplurality of NAND strings according to an embodiment.

FIGS. 3B and 3C are side cross sectional views of the memory block ofFIG. 3A along the bit line and word line directions, respectively. FIG.3B is a side cross sectional view of the device along line B-B′ in FIG.3A, while FIG. 3C is a side cross sectional view of the device alongline W-W′ in FIG. 3A.

FIGS. 4A-4G are partial side cross-sectional views of a stack ofmaterial layers over a substrate and illustrate a method of formingisolation trenches of a memory device according to an embodiment.

FIGS. 5A-5L are partial side cross-sectional views of a stack ofmaterial layers over a substrate and illustrate a method of formingconductive material control gate electrodes and source lines for athree-dimensional memory device according to an embodiment.

FIGS. 6A-6K are partial side cross-sectional views of a stack ofmaterial layers over a substrate and illustrate a method of formingconductive material control gate electrodes and source lines for athree-dimensional memory device according to another embodiment.

DETAILED DESCRIPTION

The embodiments of the invention provide a method for fabricating asemiconductor device, such as a three dimensional monolithic memoryarray comprising a plurality of NAND memory strings.

A monolithic three dimensional memory array is one in which multiplememory levels are formed above a single substrate, such as asemiconductor wafer, with no intervening substrates. The term“monolithic” means that layers of each level of the array are directlydeposited on the layers of each underlying level of the array. Incontrast, two dimensional arrays may be formed separately and thenpackaged together to form a non-monolithic memory device. For example,non-monolithic stacked memories have been constructed by forming memorylevels on separate substrates and adhering the memory levels atop eachother, as in Leedy, U.S. Pat. No. 5,915,167, titled “Three DimensionalStructure Memory.” The substrates may be thinned or removed from thememory levels before bonding, but as the memory levels are initiallyformed over separate substrates, such memories are not true monolithicthree dimensional memory arrays.

In some embodiments, a monolithic three dimensional NAND string 150comprises a semiconductor channel 1 having at least one end portionextending substantially perpendicular to a major surface 100 a of asubstrate 100, as shown in FIGS. 1A, 1C and 2. For example, thesemiconductor channel 1 may have a pillar shape extending through aplurality of memory device levels (Level A, Level B, etc.) and theentire pillar-shaped semiconductor channel in the memory device levelsextends substantially perpendicularly to the major surface 100 a of thesubstrate 100, as shown in FIGS. 1A, 1C and 2. The channels 1 may beelectrically connected to first and second (e.g., source and drain)electrodes 102, 103 which are schematically shown in FIGS. 1A and 1C.The first (e.g., source) electrode 102 may connect to the bottom of thechannel 1 and the second (e.g., drain electrode 103) may connect to thetop of the channel 1. The NAND string 150 may further include drain-sideand source-side select or access transistors (not shown in FIGS. 1A-2for clarity) which may be located above and below the memory levels ofthe NAND string 150, respectively.

In some embodiments, the semiconductor channel 1 may be a filledfeature, as shown in FIGS. 1C and 1D. In some other embodiments, thesemiconductor channel 1 may be hollow, for example a hollow cylinderfilled with an insulating fill material 2, as shown in FIGS. 1A and 1B.In these embodiments, an insulating fill material 2 may be formed tofill the hollow part surrounded by the semiconductor channel 1.

A memory device 180 may comprise a plurality of NAND strings 150 formedin a stack 120 of material layers over the substrate 100, as shown inFIG. 2. The substrate 100 can be any semiconducting substrate known inthe art, such as monocrystal line silicon, IV-IV compounds such assilicon-germanium or silicon-germanium-carbon, Ill-V compounds, II-VIcompounds, epitaxial layers over such substrates, or any othersemiconducting or non-semiconducting material, such as silicon oxide,glass, plastic, metal or ceramic substrate. The substrate 100 mayinclude integrated circuits fabricated thereon, such as driver circuitsfor a memory device.

Any suitable semiconductor materials can be used for semiconductorchannel 1, for example silicon, germanium, silicon germanium, or othercompound semiconductor materials, such as III-V, II-VI, or conductive orsemiconductive oxides, etc. The semiconductor material may be amorphous,polycrystalline or single crystal. The semiconductor channel materialmay be formed by any suitable deposition methods. For example, in oneembodiment, the semiconductor channel material is deposited by lowpressure chemical vapor deposition (LPCVD). In some other embodiments,the semiconductor channel material may be a recyrstallizedpolycrystalline semiconductor material formed by recrystallizing aninitially deposited amorphous semiconductor material.

The insulating fill material 2 may comprise any electrically insulatingmaterial, such as silicon oxide, silicon nitride, silicon oxynitride, orother high-k insulating materials.

The monolithic three dimensional NAND strings 150 further comprise aplurality of control gate electrodes 3 as shown in FIGS. 1A-2. Thecontrol gate electrodes 3 may comprise a portion having a strip shapeextending substantially parallel to the major surface 100 a of thesubstrate 100. The plurality of control gate electrodes 3 comprise atleast a first control gate electrode 3 a located in a first device level(e.g., memory device level A) and a second control gate electrode 3 blocated in a second device level (e.g., memory device level B) locatedover the major surface 100 a of the substrate 100 and below the devicelevel A. The control gate material may comprise any one or more suitableconductive or semiconductor control gate material known in the art, suchas doped polysilicon, tungsten, tungsten nitride, copper, aluminum,tantalum, titanium, cobalt, titanium nitride, alloys thereof orcombination of these materials. For example, the control gate materialin FIGS. 1A-1D may comprise a conductive metal or metal alloy, such astungsten, titanium nitride, and/or tungsten nitride, while the controlgate material in FIG. 2 may comprise doped polysilicon.

A blocking dielectric 7 is located adjacent to the control gate(s) 3 andmay surround the control gate 3, as shown in FIGS. 1A and 1C.Alternatively, a straight blocking dielectric layer 7 may be locatedonly adjacent to an edge (i.e., minor surface) of each control gate 3,as shown in FIG. 2. The blocking dielectric 7 may comprise one or morelayers having plurality of blocking dielectric segments located incontact with a respective one of the plurality of control gateelectrodes 3. Alternatively, the blocking dielectric 7 may comprise oneor more continuous layers which extend the entire length of the memorycell portion of the NAND string 150, as shown in FIG. 2.

The monolithic three dimensional NAND string also comprise a chargestorage region 9. The charge storage region 9 may comprise one or morecontinuous layers which extend the entire length of the memory cellportion of the NAND string as shown in FIG. 2. For example, the chargestorage region 9 may comprise an insulating charge trapping material,such as a silicon nitride layer. Alternatively, the charge storageregion may comprise a plurality of discrete charge storage regions orsegments 9 located between the blocking dielectric 7 and the channel I,as shown in FIGS. 1A and 1C. The discrete charge storage regions 9 maycomprise a plurality of vertically spaced apart, conductive (e.g., metalsuch as tungsten, molybdenum, tantalum, titanium, platinum, ruthenium,and alloys thereof, or a metal silicide such as tungsten silicide,molybdenum silicide, tantalum silicide, titanium silicide, nickelsilicide, cobalt silicide, or a combination thereof), or semiconductor(e.g., polysilicon) floating gates. Alternatively, the discrete chargestorage regions 9 may comprise an insulating charge trapping material,such as silicon nitride segments. Alternatively, the charge storageregion 9 may comprise conductive nanoparticles, such as metalnanoparticles, for example ruthenium nanoparticles.

The tunnel dielectric 11 of the monolithic three dimensional NAND stringis located between charge storage region 9 and the semiconductor channel1.

The blocking dielectric 7 and the tunnel dielectric 11 may beindependently selected from any one or more same or differentelectrically insulating materials, such as silicon oxide, siliconnitride, silicon oxynitride, or other insulating materials, such asmetal oxide materials, for example aluminum oxide or hafnium oxide. Theblocking dielectric 7 and/or the tunnel dielectric 11 may includemultiple layers of silicon oxide, silicon nitride and/or siliconoxynitride (e.g., ONO layers).

In various embodiments, the three-dimensional NAND string 150 may have agenerally pillar shape that extends substantially perpendicular to themajor surface of the substrate 100, with a first (e.g., drain) electrode103 that connects to the NAND string 150 at the top of the NAND string150 (i.e., distal to the substrate 100) and a second (e.g., source)electrode 102 that connects to the NAND string 150 at the bottom of theNAND string 150 (i.e., proximate to the substrate 100). In embodiments,each NAND string 150 may have a first select or access transistor (e.g.,a drain-side select gate transistor) located above the memory levels ofthe NAND string 150, and a second select or access transistor (e.g., asource-side select gate transistor) located below the memory levels ofthe NAND string 150.

In embodiments, the generally vertically-extending semiconductor channel1 of each NAND string 150 (i.e. first semiconductor channel portion) maybe electrically coupled to a second semiconductor channel portionlocated over or within the substrate 100 and extending in a generallyhorizontal direction (i.e., parallel to the major surface 100 a of thesubstrate 100). One or more trenches may be formed through the stack 120proximate to the NAND strings 150, and electrically conductive sourceline(s) may be formed within the trench(es) and in electrical contactwith the generally horizontally-extending second portions of thesemiconductor channels.

FIGS. 3A-3C illustrate a memory block 400 containing an array of aplurality of vertical NAND strings 150 according to one embodiment ofthe invention. Each string includes the lower 50 (e.g., source) andupper 60 (e.g., drain) select gate device levels located below and abovethe memory device levels 70, respectively, as shown in FIG. 3C. In thisembodiment, the memory block 400 comprises an active memory cell area200 containing the array of vertical NAND strings 150. The active memorycell area 200 is bordered on one side by a first isolation trench (i.e.,slit trench) 84 a and on the opposite side by a second isolation trench(i.e., slit trench) 84 b. On the left side of the active memory cellarea 200 in FIG. 3A is a stepped word line contact region 300.

FIG. 3A is a top cross sectional view of a memory block of an embodimentof the invention. FIGS. 3B and 3C are side cross sectional views of thememory block of FIG. 3A along the bit line and word line directions,respectively. FIG. 3B is a side cross sectional view of the device alongline B-B′ in FIG. 3A, while FIG. 3C is a side cross sectional view ofthe device along line W-W′ in FIG. 3A.

As shown in FIGS. 3B and 3C, the lower (e.g., source) select gate devicelevel 50 includes a select gate electrode 204 over the substrate 100that extends generally parallel to the major surface 100 a of thesubstrate 100. A plurality of protrusions 1B comprising a semiconductorchannel material extend in a generally vertical direction from the majorsurface 100 a of the substrate such that the select gate electrode 204is adjacent to at least two opposing side surfaces of each protrusion1B. A gate insulating layer 206 extends generally parallel to the majorsurface of the substrate 100 a between the select gate electrode 204 andthe substrate 100 and also extends generally perpendicular to the majorsurface 100 a of the substrate 100 between the select gate electrode 204and the side surfaces of the protrusions 1B. Each NAND string 150 islocated above a protrusion 1B (i.e., optional third semiconductorchannel portion), such that the generally vertically-extending channel 1(i.e., first semiconductor channel portion) of each NAND string 150electrically contacts the top surface of a protrusion 1B.

The select gate electrode 204 may comprise any suitable electricallyconductive material, such as heavily doped semiconductor (e.g., heavilydoped polysilicon), a metal or metal alloy. The select gate electrode204 preferably comprises a metal or metal alloy, such as tungsten,copper, aluminum, tantalum, titanium, cobalt, titanium nitride, alloysthereof or combination of these materials. In one non-limitingembodiment, the select gate electrode 204 comprises a titanium nitrideliner peripheral portion contacting the gate insulating layer 206, and atungsten central portion contacting the titanium nitride linerperipheral portion. The gate insulating layer 206 may comprise asuitable insulating material, such as silicon oxide. The protrusion 1Bmay comprise a suitable semiconductor material, such as anepitaxially-grown single crystal semiconductor material (e.g., silicon)and/or a polycrystalline semiconductor (e.g., silicon) material that isrecrystallized by thermal treatment or laser annealing to form singlecrystal or large grain polycrystalline semiconductor material.

Various fabrication methods and select gate device level 50configurations may be used. For example, a lower select gate devicelevel 50 may be fabricated as described in U.S. patent application Ser.No. 14/133,979, filed on Dec. 19, 2013, U.S. patent application Ser. No.14/225,116, filed on Mar. 25, 2014, U.S. patent application Ser. No.14/225,176, filed on Mar. 25, 2014, and/or U.S. Provisional ApplicationNo. 61/977,193, filed on Apr. 9, 2014, all of which are incorporated byreference herein for all purposes.

As is shown in FIG. 3B, additional (e.g., second) semiconductor channelportions 1C may be located over or in the substrate 100 and may extendin a direction that is generally parallel to the major surface 100 a ofthe substrate 100 (i.e., in a generally horizontal direction in FIG.3B). The additional semiconductor channel portions 1C may electricallycouple the semiconductor channel portions of the protrusions 1B to aconductive source line 102 located in an isolation trench 84 a, 84 b. Inthe embodiment of FIG. 3B, the generally horizontally-extending channelprotrusions 1C are electrically connected to an additional (i.e.,fourth) channel portion 1D that extends in a generally verticaldirection within a pillar- or rail-shaped protrusion 207 that extends ina generally perpendicular direction from the major surface 100 a of thesubstrate 100, and the source line 102 is electrically coupled to thechannel portion 1D at the top surface of the protrusion 207. In otherembodiments, the protrusion 207 may be omitted, and the source line 102may directly contact the generally horizontally-extending channelportion 1C (i.e., second semiconductor channel portion). Thus, thesource lines 102 may be electrically connected to the bottom portions ofthe semiconductor channels 1 (i.e., first semiconductor channelportions) in the NAND strings 150 via the second, third and optionallyfourth semiconductor channel portions 1C, 1B and 1D.

The upper select gate device level 60 contains the drain side selectgate transistor (not shown for clarity) located over the NAND memorycell region in memory device levels 70. As discussed above, each NANDstring 150 contains a NAND memory cell region in the memory devicelevels 70 which includes the semiconductor channel 1 which extendsperpendicular to the major surface 100 a of the substrate 100. A bottomportion of the semiconductor channel 1 contacts the protrusion 1B in thelower (i.e., source-side) select gate device level 50.

As illustrated in FIG. 3C, each NAND string 150 contains a plurality ofcontrol gate electrodes 3 which extend substantially parallel to themajor surface 100 a of the substrate 100 in the memory device levels 70from the memory region 200 to the stepped word line contact region 300.The portions of the control gate electrodes 3 which extend into region300 may be referred to as “word lines” herein. The drain line 203electrically contacts an upper portion of the semiconductor channel 1via drain electrodes 103.

Furthermore, each NAND string 150 contains at least one memory filmwhich is located adjacent to the semiconductor channel 1 in the memorydevice levels 70. Specifically, the memory film is located between thesemiconductor channel 1 and the plurality of control gate electrodes 3.The memory film contains the tunnel dielectric 11, the charge storageregion(s) 9 (e.g., a charge trapping layer or floating gates), and theblocking dielectric 7, as described above in connection with FIGS. 1A-2.

FIG. 3A illustrates a top view of a memory block 400. The memory blockincludes an array of NAND strings including at least one row ofmonolithic three dimensional NAND strings 150 described above, a firstdielectric filled trench 84 a located on a first side of the array, anda second dielectric filled trench 84 a located on a second side of thearray opposite to the first side of the array.

The array of NAND strings may include any number of rows of NAND strings150. For example, the array shown in FIG. 3A comprises at least a 4×4array of NAND strings. In other words, the array shown in these figureshas four rows of NAND strings, and there are at least four NAND stringsin each row. The rows of NAND strings extend in the word line direction(e.g., along line W-W′ in FIG. 3A). Thus, the array in the block 400comprises first, second, third and fourth rows of NAND strings 150extending in the word line direction. As shown in FIG. 3A, thesemiconductor channels in the first and the third rows of NAND stringsare preferably offset from respective semiconductor channels in thesecond and fourth rows of NAND strings along the word line direction. Ingeneral, the semiconductor channels in the odd numbered rows of NANDstrings may be offset from respective semiconductor channels in the evennumbered rows of NAND strings along the word line direction. In oneembodiment, the semiconductor channels in the first and second rows ofNAND strings 150 extending in the word line direction (i.e., the top tworows in FIG. 3A) may be electrically connected to the conductive sourceline 102 in trench 84 a via semiconductor channels 1C extending parallelto the major surface 100 a of the substrate 100 (see FIG. 3B), and thesemiconductor channels in the third and forth rows of NAND strings 150extending in the word line direction (i.e., the bottom two rows in FIG.3A) may be electrically connected to the conductive source line 102 intrench 84 b via semiconductor channels 1C extending parallel to themajor surface 100 a of the semiconductor substrate 100.

In alternative embodiments, each memory block 400 may have an array withless than 4 rows of NAND strings, such as only one row of NAND stringsor two rows of NAND strings. Alternatively, the array in each block mayhave more than 4 rows, such as 5 to 20 rows of NAND strings, where eachrow may contain 4 to 100 NAND strings.

As shown in FIG. 3B, the control gate electrodes 3 extend in the wordline direction W-W′ which is perpendicular to the bit line directionB-B′. The control gate electrodes 3 are continuous in the array in thememory block 400. In other words, the control gate electrodes 3 have ashape of a continuous strip or sheet with discrete openings 81 (whichare referred to herein as front side openings or memory holes) whichcontain the NAND strings 150. However, the control gate electrodes 3have electrical and physical continuity in the bit line directionbetween the trenches 84 and in the word line direction throughout theblock 400. In other words, the memory holes 81 do not completely severthe continuous electrical and physical path in the control gateelectrodes from one trench 84 a to the opposite trench 84 b in eachblock.

Likewise, the select gate electrode 204 also extends in the word linedirection W-W′ which is perpendicular to the bit line direction B-B′.The select gate electrode 204 is also continuous in the array in thememory block 400. In other words, the select gate electrode 204 has ashape of a continuous strip or sheet with electrically insulatedprotrusions 1B extending through the electrode 204. However, the selectgate electrode 204 has electrical and physical continuity in the bitline direction between the gate insulating layer 206 under trenches 84a, 84 b and in the word line direction throughout the block 400. Inother words, the protrusions 1B do not completely sever the continuouselectrical and physical path in the select gate electrode from gateinsulating layer 206 adjacent to one trench 84 a to the gate insulatinglayer 206 adjacent to the opposite trench 84 b in each block 400.

Thus, the memory block 400 contains a common control gate electrode 3 ineach of the plurality of memory device levels 70 for the first, second,third and fourth rows of NAND strings shown in FIG. 3A. Likewise, thesource side select gate electrode 204 comprises a common source sideselect gate electrode for the first, second, third and fourth rows ofNAND strings in the block 400. Therefore, all of the NAND strings in thearray in each block 400 can be erased together in the same erase step.In addition, one specific memory cell can be selected by selecting(i.e., applying a current or voltage) to one select gate 204, one bitline (i.e., drain line) 203 and one word line (e.g., control gateelectrode) 3. Specifically, a particular block 400 may be selected withthe common select gate 204 in that block, the specific memory hole81/NAND string 150 may be selected with the bit line 203, and theparticular cell in one memory device level 70 in the NAND string 150 maybe selected with the word line 3.

A first source line 102 is located in the first dielectric filled trench84 a and a second source line 102 is located in the second dielectricfilled trench 84 a in each block 400, as shown in FIGS. 3A and 3B. Inthe memory device levels 70, the dielectric fill in the trench 84 maycomprise any suitable insulating layer 205, such as silicon oxide, etc.,which is located on both walls of the trench 84. The source line 102 islocated in the middle of the trench 84 and is separated from the controlgate electrodes 3 and from the select gate electrode 204 by theinsulating layer 205.

The trenches 84 a, 84 b are typically formed by etching the stack 120through a mask to expose a semiconductor channel portion (i.e., channelportion 1D in FIG. 3B, or channel portion 1B if portion 1D is omitted),forming the insulating layer 205 over the sidewalls of the trenches 84a, 84 b and forming the source lines 102 within the trenches 84 a, 84 bsuch that the source lines 102 electrically contact the exposedsemiconductor channel portion (e.g., a doped source region insemiconductor channel portion 1B or 1D). In order to minimize featurespacing and improve process margins, it is advantageous to form thetrenches using a high aspect ratio etching process. However, it isdifficult to maintain proper alignment of the trench 84 using a highaspect ratio etching process.

Various embodiments include methods of making a memory device such as amonolithic three-dimensional NAND string memory device. FIGS. 4A-4Gillustrate a method of making a memory device according to a first,non-limiting embodiment of the invention. In embodiments, the method ofFIGS. 4A-4G may reduce misalignment of high aspect ratio isolationtrenches and improve process margins.

As shown in FIG. 4A, a plurality of alternating layers 19, 21 of a firstmaterial and a second material different than the first material aredeposited over the major surface 100 a of the substrate 100 to form afirst stack 401. The layers 19, 21 may be deposited directly on themajor surface 100 a of the substrate 100, or may be deposited overanother layer or layer(s) that is formed on the substrate 100. Forexample, a lower (e.g., source) select gate device level 50 may beformed over or within the substrate 100 as described above, and thefirst stack 401 of alternating layers 19, 21 may be formed over thelower select gate device level 50 as shown in FIG. 4A.

The layers 19, 21 of the first stack 401 may be deposited by anysuitable deposition method, such as sputtering, CVD, PECVD, MBE, ALD,etc. The layers 19, 21 may be 6 to 100 nm thick. In this embodiment, thefirst layers 19 comprise an electrically insulating material. Anysuitable insulating material may be used, such as silicon oxide, siliconnitride, silicon oxynitride, a high-k dielectric (e.g., aluminum oxide,hafnium oxide, etc. or an organic insulating material). The secondlayers 21 may comprise a semiconductor material (e.g., silicon, such aspolysilicon). In one embodiment, layers 19 comprise silicon oxide andlayers 21 comprise polysilicon.

The deposition of layers 19, 21 is followed by etching the first stack401 to form at least one first trench 403. The first trench(es) 403 maybe formed in the locations of the future isolation trenches 84 that willdefine the active memory cells areas as described above. The firsttrenches 403 may be formed by photolithography and etching, as follows.First, a mask 405 is formed over the first stack 401 and patterned toform openings 407 exposing the stack 401, as shown in FIG. 4A. Mask 405may comprise any suitable material, such as one or more layer ofphotoresist and/or hard mask material (e.g., photoresist over siliconnitride and amorphous carbon layers). Then, the first stack 401 may beetched (e.g., using reactive ion etching (RIE)) to form the firsttrenches 403 in the stack through the openings 407 in mask 405, as shownin FIG. 4B. The first trench 403 may preferably extend through thealternating layers 19, 21 of the first stack 400, and may optionallyextend through a lower select gate device layer 50 to the substrate 100,as shown in FIG. 4B. The mask 405 may then be removed.

The first trenches 403 may each comprise a pair of first trenchsidewalls 409 extending from a first end 411 of the trench 403 (i.e.,the bottom 410 of the trench 403) proximate to the substrate 100 to asecond end 413 distal to the substrate 100. The distance between thesidewalls 409 defines a width of the trench 403, and the width of thetrench 403 may decrease between the second end 413 and the first end 411of the trench 403 as shown in FIG. 4B. The first trench 403 may have awidth, W₁, at the second end 413 of the trench 403 (i.e., distal to thesubstrate 100).

The first trenches 403 may be filled with a sacrificial material 415, asshown in FIG. 4C. The sacrificial material 415 may comprise a dielectricmaterial, such as silicon nitride or silicon oxide. Alternatively or inaddition, the sacrificial material 415 may comprise a carbon material(e.g., undoped carbon, doped carbon, amorphous carbon and/orpolycrystalline carbon) and/or a semiconductor material, such as undopedpolysilicon. The sacrificial material 415 may be planarized, such as bychemical mechanical polishing (CMP) or an etch-back process, to make thesacrificial material 415 planar with the top surface of the first stack401, as shown in FIG. 4C.

A second stack 417 of alternating layers 19, 21 of the first materialand the second material may be formed over the first stack 401 and thesacrificial material 415, as shown in FIG. 4D. The alternating layers19, 21 of the second stack 417 may be the same as the layers 19, 21 inthe first stack 401 such that the first and second stacks 401, 417 forma continuous stack 120 of alternating material layers 19, 21 over thesubstrate 100.

A plurality of NAND memory strings 150 may be formed in the continuousstack 120 by forming a plurality of front side openings/memory holes 81extending through the continuous stack 120 in a direction that issubstantially perpendicular to the major surface 100 a of the substrate100, forming at least one memory film (e.g., a tunnel dielectric layer7, a charge trap/floating gate layer 9 and/or a blocking dielectriclayer 11 as shown in FIGS. 1A-2) within each front side opening/memoryhole 81, and forming a semiconductor channel 1 within each front sideopening/memory hole 81. FIG. 4E schematically illustrates four NANDmemory strings 150 formed in memory holes 81 that are located betweenthe first trenches 403. Each of the NAND memory strings 150 may besubstantially as described above in connection with FIGS. 1A-3C. TheNAND memory strings 150 may have an offset configuration such as shownin FIG. 3A in which semiconductor channels in the first and the thirdrows of NAND strings are preferably offset from respective semiconductorchannels in the second and fourth rows of NAND strings along the wordline direction. Thus, the NAND memory strings 150 shown in FIG. 4E maybe taken along line A-A′ in FIG. 3A.

The NAND memory strings 150 in FIG. 4E may lack conductive control gateelectrodes 3, which may be formed in a subsequent control gatereplacement process, as described further below. Various methods forforming NAND memory strings 150 in a stack 120 of alternating materiallayers are described in the previously-mentioned U.S. patent applicationSer. Nos. 14/133,979, 14/225,116, 14/225,176 and 61/977,193, which areincorporated by reference herein for all purposes.

The stack 120 may be etched to form at least one second trench 419. Thesecond trenches 419 may be formed in the locations of the futureisolation trenches 84 that will define the active memory cell areas asdescribed above. Thus, the second trenches 419 may be formed above andsubstantially aligned with the first trenches 403 that are filled withsacrificial material 415. In FIGS. 4E-4G, the second trenches 419 areformed following the formation of the NAND strings 150 in the stack 120.In other embodiments, the second trenches 419 may be formed before thememory holes 81 and NAND strings 150 are formed.

The second trenches 419 may be formed by photolithography and etching,as follows. First, a mask 421 (e.g., photoresist and/or hard maskmaterial) may formed over the continuous stack 120 and patterned to formopenings 423 exposing the top surface of the stack 120, as shown in FIG.4E. The mask 421 may be substantially identical to the mask 405 used toform the first trenches 403 as described above (see FIG. 4A), and thelocation and size of the openings 423 may substantially correspond tothe location and size of the openings 407 in mask 405. In general, thelocations of the openings 423 in mask 421 correspond to the locations ofthe openings 407 in the mask 405 used to form the first trenches 403. Insome embodiments, the size of the openings 423 in mask 421 may bedifferent than the size of the openings 407 in mask 405 (e.g., openings423 may be slightly larger or smaller than openings 407).

Then, the continuous stack 120 may be etched (e.g., using reactive ionetching (RIE)) through the openings 423 in mask 421, stopping at thesacrificial material 415 in the first trenches 403, to form the secondtrenches 419, as shown in FIG. 4F. The mask 421 may then be removed, asshown in FIG. 4G.

As shown in FIG. 4F, the second trenches 419 each comprise a pair ofsecond trench sidewalls 425 extending from a first (e.g., bottom) end427 of the trench 419 proximate to the second (e.g., top) end 413 of thefirst trench 403 to a second (e.g., top) end 429 distal to the secondend 413 of the first trench 403. The distance between the second trenchsidewalls 425 defines a width of the second trench 419, and the width ofthe second trench 419 may decrease between the second end 429 and thefirst end 427 of the trench 419 as shown in FIG. 4F. The second trench419 may have a width, W₂, at the first end 427 of the second trench 419(i.e., adjacent to the sacrificial material 415 at the second end 413 ofthe first trench 403). The width W₁ of the first trench 403 at thesecond end 413 of the first trench 403 (see FIG. 4B) may be greater thanthe width W₂ of the second trench 419 at the first end 427 of the secondtrench 419.

In FIG. 4G, the sacrificial material 415 is removed from the firsttrenches 403 to form continuous trenches 84 a, 84 b that extend throughthe continuous stack 120 (i.e., through the first stack 401 and thesecond stack 417). The sacrificial material 415 may be removed using anysuitable process, such as by selectively etching the sacrificialmaterial 415 through the second trench 419 to remove the sacrificialmaterial 415 (e.g., via a wet chemical etch or ashing if the sacrificialmaterial 415 comprises carbon) from the first trench 403 while leavingthe sidewalls 409 and bottom 410 of the first trench 403 substantiallyintact. The pairs of first and second trench sidewalls 409, 425 may formthe sidewalls of the respective continuous trenches 84 a, 84 b anddefine a width of the continuous trench, W_(C). Each continuous trench84 a, 84 b may have a depth dimension that extends along a directionbetween a top 431 of the continuous trench 84 a, 84 b (i.e., proximateto the upper surface of the continuous stack 120) and a bottom 433 ofthe continuous trench 84 a, 84 b (i.e., proximate to the substrate 100).The width W_(C) of the trench 84 a, 84 b may be substantially constantor may decrease from the top 431 of the trench to a first depth (i.e.,corresponding to the first end 427 of the second trench 419 in FIG. 4F)and may increase between the first depth and a second depth (i.e.,corresponding to the second end 413 of the first trench 403 in FIG. 4B),where the second depth is closer to the bottom 433 of the trench 84 a,84 b than the first depth. In other words, the width W_(C) of thecontinuous trench 84 a, 84 b initially decreases or remains constantalong the depth direction towards the bottom 410/433 of the trench, andthen increases along the depth direction, as shown in FIG. 4G. The widthof the trench may increase at the transition between the second trench419 formed through the second stack 417 and the first trench 403 formedthrough the first stack 401. Thus, at a first depth, the width W_(C) ofthe continuous trench may be W₂ (i.e., corresponding to the width of thesecond trench 419 at the first end 427 of the second trench 419 shown inFIG. 4F) and at a second depth closer to the substrate 100 than thefirst depth, the width W_(C) of the continuous trench may increase to W₁(i.e., corresponding to the width of the first trench 403 at the secondend 413of the first trench 403 shown in FIG. 4B).

FIGS. 5A-5L illustrate additional processing steps that may be performedto at least partially remove the layers 21 of the second material fromthe stack 120 and form control gate electrodes 3 and conductive sourcelines 102 for a vertical NAND memory string 150. FIG. 5A illustrates amemory device that is similar to the memory device shown in FIG. 4G. Thedevice of FIG. 5A includes a stack 120 of alternating layers of a firstmaterial 19 and a second material 21 over the major surface 100 a of asubstrate 100, and a plurality of NAND memory strings 150 extendingthrough the stack 120 in a direction that is substantially perpendicularto the major surface 100 a of the substrate 100. FIG. 5A alsoillustrates a pair of continuous trenches 84 a, 84 b that extend throughthe stack 120 to the substrate 100, wherein the trenches 84 a, 84 bdefine an active memory cell area 200 between the trenches, and the NANDstrings 150 are located in the active memory cell area 200. The trenches84 a, 84 b in this embodiment are shaped as described above withreference to FIG. 4G (i.e., a width of the trench increases along thedepth of the trench, as described above), although it will be understoodthat the trenches 84 a, 84 b, may have a different shape (e.g., suchthat the width of the trench is constant or decreases over the entiredepth of the trench).

FIG. 5A illustrates the NAND strings 150 in the respective memory holes81 including at least one memory film 501 (e.g., a tunnel dielectriclayer 7, a charge trap/floating gate layer 9 and/or a blockingdielectric layer 11 as shown in FIGS. 1A-2) extending over the sidewallsof the memory holes 81 and the semiconductor channel 1 extendinggenerally perpendicular to the major surface 100 a of the substrate 100and over the at least one memory film 501, such that the at least onememory film 501 is located between the semiconductor channel 1 and thesidewall of the memory hole 81. FIG. 5A further illustrates aninsulating material core 2 (see FIGS. 1A-1B) within each memory hole 81.An insulating cover layer 502 (e.g., silicon oxide) may be located overthe stack 120.

FIG. 5A further illustrates a second semiconductor channel portion 503that extends in a generally horizontal direction parallel to the majorsurface 100 a of the substrate 100. In this embodiment, the secondsemiconductor channel portion 503 is located within the substrate 100,and is located between at least one memory film 501 and the insulatingmaterial core 2. In other embodiments, the second semiconductor channelportion 503 may be located on or over the major surface 100 a of thesubstrate, and the at least one memory film 501 and the insulatingmaterial core 2 may not extend in a generally horizontal directionadjacent to the second semiconductor channel portion 503. As discussedabove, the vertically extending semiconductor channel 1 (i.e., firstsemiconductor channel portion) located in a memory hole 81 may beelectrically connected to the horizontally extending secondsemiconductor channel portion 503 in a location of a lower (e.g.,source) select gate transistor. In other words, the select gatetransistor may be located between the second semiconductor channelportion 503 and the memory levels of the NAND string 150. For clarity,the lower (e.g., source) select gate transistors are not shown in FIGS.5A-5L.

In FIG. 5B, the substrate 100 may be etched through the trenches 84 a,84 b, such as via reactive ion etching (RIE), to expose the at least onememory film 501 located over a region 505 of the second semiconductorchannel portion 503 at the bottom of each trench 84 a, 84 b. The regions505 may be regions of the second semiconductor channel portions 503 thatwill later be implanted to form doped source regions which contactconductive source lines formed in the trenches 84 a, 84 b, as discussedbelow.

In FIG. 5C, a protective liner material 507 is formed over at least thethe sidewalls of the trenches 84 a, 84 b. The protective liner material507 may comprise silicon nitride, for example, and may be deposited suchthat the protective liner material 507 preferentially forms on thesidewalls of the trenches 84 a, 84 b but not on the bottoms of thetrenches 84 a, 84 b. The protective liner material 507 may also beformed over the insulating cover layer 502 on the top of the stack 120,as shown in FIG. 5C. The protective liner material 507 may remain overand protect the sidewalls of the trenches 84 a, 84 b from being etchedduring a subsequent etching step, similar to sidewall spacer formationduring anisotropic etching.

In FIG. 5D, the at least one memory film 501 and optionally anyprotective liner material 507 at the bottom 410 of the trenches 84 a, 84b is etched to expose the regions 505 of the second semiconductorchannel portion 503. An anisotropic etching process may be used, such asreactive ion etching (RIE), so that generally horizontally extendingmaterials, such as the at least one memory film 501 at the bottom of thetrenches 84 a, 84 b and the protective liner material 507 over theinsulating cover layer 502, are preferentially etched relative togenerally vertically-extending materials, such as the protective linermaterial 507 over the sidewalls of the trenches 84 a, 84 b. Thus, theprotective liner material 507 may protect the trench sidewalls frometching damage during the etching step.

In FIG. 5E, ion implantation is performed through the trenches 84 a, 84b to form source regions 508 in the exposed regions 505 of the secondsemiconductor channel portion 503. The ion implantation may be followedby an activation anneal step. The source regions 508 may be dopedopposite conductivity type (e.g., n-type) from the conductivity type(e.g., p-type) of the second semiconductor channel portion 503.

In FIG. 5F, the protective liner material 507 may be removed from thetrench sidewalls, such as via a selective wet chemical etch.

In FIG. 5G, a bottom portion of each trench 84 a, 84 b is filled with aprotective material 509. The protective material 509 may comprise aninsulating material, such as an oxide material (e.g., silicon oxide),that covers the source regions 508 of the second semiconductor channelportion 503 and partially fills the trenches 84 a, 84 b at the bottomsof the trenches, but is preferably not located in the trenches 84 a, 84b adjacent to the memory levels 70 (i.e., the portion of the stack 120in which control gate electrodes are subsequently formed). Inembodiments, the protective material 509 may be deposited in thetrenches 84 a, 84 b such that the material 509 preferentially depositson the bottom of the trench but not on the sidewalls of the trenchadjacent to the memory levels 70. Alternately, the protective material509 may be deposited such that the protective material 509 overfills thebottom portions of the trenches 84 a, 84 b (i.e., a portion of theprotective material 509 is located adjacent to the memory levels 70) andthe protective material 509 may then be etched back or recessed throughthe trenches 84 a, 84 b to remove the protective material 509 adjacentto the memory levels 70, leaving only the bottom portions of thetrenches 84 a, 84 b filled with the protective material 509.

In FIG. 5H, portions of the second material layers 21 of the stack 120are removed through the trenches 84 a, 84 b form back side recesses 511between the first material layers 19. The portions of the secondmaterial layers 21 may be removed by selective etching, such aspolysilicon selective etching which removes the polysilicon material ofthe second material layers 21 but does not remove the silicon oxidematerial of the first material layers 19. The protective material 509 atthe bottoms of the trenches 84 a, 84 b may protect the source region 508in the second semiconductor channel portion 503 from being etched.

In the embodiment of FIG. 5H, only a portion of the second materiallayers 21 are selectively removed from the stack 120 during the etching.Preferably, the second material layers 21 are completely removed in thememory levels 70 from the active memory cell area 200 defined betweenthe trenches 84 a, 84 b. Within the active memory cell area 200, thesecond material layers 21 may be completely removed by selectiveetching, and the selective etch may stop on the at least one memory film501 (e.g., an oxide blocking dielectric 7, such as a silicon oxideblocking dielectric, as shown in FIGS. 1A-2) that extends vertically inthe memory openings 81. Outside of the active memory cell area 200(i.e., on the sides of the trenches 84 a, 84 b opposite the activememory cell area 200), at least a portion of the second material layers21 is not removed, as shown in FIG. 5H.

Electrically conductive control gate electrodes 3 may then be formed inthe back side recesses 511 through the trenches 84 a, 84 b, as shown inFIG. 5I. The control gate electrode 3 material may comprise any suitablematerial described above with respect to FIGS. 1A-2. For example, thematerial may comprise a TiN liner and tungsten gate material. Theelectrodes 3 may be formed by forming the electrically conductivecontrol gate electrode material to partially or completely fill thetrenches 84 a, 84 b and to fill the back side recesses 511 such that thecontrol gate electrode 3 material contacts the at least one memory film501 (i.e., blocking dielectric 7) along the sidewalls of the memoryopenings 81. The electrically conductive control gate material may beformed using any suitable technique, such as by chemical vapordeposition (CVD). The electrode material may then be removed from thetrenches 84 a, 84 b (e.g., using anisotropic etching) without removingthe material forming the electrodes 3 from the recesses 511.

Thus, within the active memory cell area 200, the stack 120 comprisesalternating layers 19, 3 of the first material (e.g., an insulatingmaterial, such as silicon oxide) and a conductive material (e.g., ametal, metal silicide or metal nitride material, such as tungsten,tungsten silicide, tungsten nitride, titanium, titanium silicide ortitanium nitride). The conductive material forms a plurality ofconductive control gate electrodes 3 that extend adjacent to the atleast one memory film 501 in each of the NAND memory strings 150.Outside of the active memory cell area 200, at least a portion of thestack 120 comprises alternating layers 19, 21 of the first material andan additional material that is different from the first material and theconductive gate electrode material (e.g., the additional material may bea semiconductor material, such as polysilicon).

Then, as shown in FIG. 5J, an insulating layer 517, such as a siliconoxide or silicon nitride layer, is formed over the stack 120, includingover the insulating cover layer 502 and within the trenches 84 a, 84 b,including over the trench sidewalls and over the protective material 509at the bottom of the trenches 84 a, 84 b. In FIG. 5K, the insulatinglayer 517 and the protective material 509 are etched from the bottoms ofthe trenches 84 a, 84 b to expose the source regions 508 of the secondsemiconductor channel portion 503. Portions of the insulating layer 517and the protective material 509 remain over the sidewalls of thetrenches 84 a, 84 b, as shown in FIG. 5K.

In FIG. 5L, a conductive source line 102 (e.g., a metal or metal nitrideline, such as W, Ti, TiN, etc.) is then formed over the insulating layer517 and the protective material 509 in each of the trenches 84 a, 84 bsuch that the source line 120 contacts a source region 508 of the secondsemiconductor channel portion 503. The source line 120 is electricallyisolated from the control gate electrodes 3 by the insulating layer 517and protective material 509 along the sidewalls of the trench 84 a, 84b.

FIGS. 6A-6K illustrate an alternative method of forming control gateelectrodes 3 and conductive source lines 102 for a vertical NAND memorystring 150. FIG. 6A corresponds to FIG. 5A, and shows a plurality ofNAND memory strings 150 extending through a stack 120 in a directionthat is substantially perpendicular to the major surface 100 a of thesubstrate 100. FIG. 6A also shows a pair of continuous trenches 84 a, 84b that extend through the stack 120 to the substrate 100, wherein thetrenches 84 a, 84 b define an active memory cell area 200 between thetrenches, and the NAND strings 150 are located in the active memory cellarea 200. The trenches 84 a, 84 b in this embodiment are shaped asdescribed above with reference to FIG. 4G (i.e., a width of the trenchincreases along the depth of the trench, as described above), althoughit will be understood that the trenches 84 a, 84 b, may have a differentshape (e.g., such that the width of the trench is constant or decreasesover the entire depth of the trench).

The stack 120 in FIG. 6A differs from the stack 120 in FIG. 5A in thatinstead of alternating layers of a first insulating (e.g., an oxide,such as silicon oxide) material 19 and a second semiconductor (e.g.,silicon, such as polysilicon) material 21 (e.g., an OPOP stack) shown inFIG. 5A, the stack 120 of FIG. 6A includes alternating layers of a firstinsulating (e.g., an oxide, such as silicon oxide) material 19 and asecond sacrificial (e.g., a nitride, such as silicon nitride) material121 different from the first material (e.g., an ONON stack).

FIGS. 6B-6E substantially correspond to FIGS. 5B-5E described above, andillustrate etching the substrate 100 through the trenches 84 a, 84 b toexpose the at least one memory film 501 located over a region 505 of thesecond semiconductor channel portion 503 at the bottom of each trench 84a, 84 b (FIG. 6B), forming a protective liner material 507 (e.g.,silicon nitride) over at least the sidewalls of the trenches 84 a, 84 b(FIG. 6C), etching the at least one memory film 501 through the trenches84 a, 84 b to expose regions 505 of the second semiconductor channelportion 503 while the protective liner material 507 protects the trenchsidewalls from etching damage (FIG. 6D), and performing ion implantationthrough the trenches 84 a, 84 b to form source regions 508 in theexposed regions 505 of the second semiconductor channel portion 503(FIG. 6E). Preferably, the liner material 507 comprises the samematerial (e.g., silicon nitride) as the second sacrificial layer 121material.

In the embodiment of FIGS. 6A-6K, the separate step of removing theprotective liner material 507 from the trenches 84 a, 84 c (e.g., viawet etching) as shown in FIG. 5E prior to filling the bottoms of thetrenches 84 a, 84 b with a protective material 509 may be omitted. Thus,the protective material 509 may be formed in the bottoms of the trenches84 a, 84 b and over the exposed source regions 508 while the protectiveliner material 507 remains over the sidewalls of the trenches 84 a, 84b, as shown in FIG. 6F. The protective material 509 may be an oxidematerial (e.g., silicon oxide) which is different from the nitridematerial of layers 121 and 507 and may be deposited over the bottoms ofthe trenches 84 a, 84 b via CVD using a tetraethyl orthosilicate (TEOS)source.

In FIG. 6G, the protective liner material 507 and the second layers 121of sacrificial material are removed through the trenches 84 a, 84 b formback side recesses 511 between the first material layers 19. Theprotective liner material 507 and the second layers 12I may becompletely removed by selective etching, such as silicon nitrideselective etching which removes the silicon nitride material of theprotective liner material 507 and the second layers 121 in the same stepbut does not remove the silicon oxide material of the first materiallayers 19 and the protective material 509. The protective material 509at the bottoms of the trenches 84 a, 84 b may protect the source region508 in the second semiconductor channel portion 503 from being etched.

Unlike in the embodiment of FIGS. 5A-5L in which only a portion of thesecond material layers 21 are selectively removed from the stack 120during the etching, preferably the second layers 121 of sacrificialmaterial are completely removed from the stack 120, including from theactive memory cell area 200 defined between the trenches 84 a, 84 aswell as from outside of the active memory cell area 200, as shown inFIG. 6G. In embodiments, one or more support columns of a suitablematerial (e.g., an insulating material, such as silicon oxide or anothermaterial that is not etched with the sacrificial material) may be formedthrough the stack 120 (e.g., outside of the view of FIG. 6G) to supportthe stack layers after the sacrificial layer 121 are removed. Forclarity, the one or more support column is not shown in FIG. 6G. Anexemplary support column and method of fabricating a support column isdescribed and illustrated with reference to FIGS. 44-46 of U.S.application Ser. No. 14/133,979, which is incorporated by referenceherein. It will be understood that one or more support columns may alsobe formed to support the stack layers after the second material layers21 are partially etched in the embodiment of 5A-5L.

Thus, as shown in FIG. 6G, the second layers 121 of sacrificial materialare removed both from within the active memory cell area 200 and fromoutside of the active memory cell area 200 (i.e., on the sides of thetrenches 84 a, 84 b opposite the active memory cell area 200). Thesacrificial material layers 121 may be removed by selective etching, asdescribed above, and within the active memory cell area 200, the etchingmay stop on the at least one memory film 501 (e.g., an oxide blockingdielectric 7, such as a silicon oxide blocking dielectric, as shown inFIGS. 1A-2) that extends vertically in the memory openings 81.

Electrically conductive control gate electrodes 3 may then be formed inthe back side recesses 511 through the trenches 84 a, 84 b, as shown inFIG. 6H. The electrically conductive control gate electrodes 3 may beformed as described above with reference to FIG. 5I. Thus, both withinthe active memory cell area 200 and outside of the active memory cellarea 200, the stack 120 comprises alternating layers 19, 3 of the firstmaterial (e.g., an insulating material, such as silicon oxide) and aconductive material (e.g., a metal, metal silicide or metal nitridematerial, such as tungsten, tungsten silicide, tungsten nitride,titanium, titanium silicide or titanium nitride). Within the activememory cell area 200, the conductive material forms a plurality ofconductive control gate electrodes 3 that extend adjacent to the atleast one memory film 501 in the memory levels 70 of each of the NANDmemory strings 150. Outside of the active memory cell area 200, thelayers of sacrificial material 121 are completely removed and the stack120 comprises alternating layers 19, 3 of the first material and theconductive material.

Although the method of control gate electrode 3 formation of FIGS. 6A-6His described with reference to a stack 120 comprising alternating layers19 of a first insulating (e.g., an oxide, such as silicon oxide) and asecond sacrificial (e.g., a nitride, such as silicon nitride) material121 (e.g., an ONON stack), it will be understood that the method mayalso be used with a stack 120 that includes alternating layers of afirst insulating (e.g., an oxide, such as silicon oxide) material 19 anda second semiconductor (e.g., silicon, such as polysilicon) material 21(e.g., an OPOP stack), as shown in FIG. 5A. Thus, the second layers 21of semiconductor material (e.g., polysilicon) may be completely removedfrom the stack 120 and replaced with a conductive material (e.g., ametal or metal nitride) that forms a control gate electrode 3 for theNAND memory strings 150.

FIG. 61 substantially corresponds to FIG. 5J, described above, andillustrates the formation of an insulating layer 517, such as a siliconoxide or silicon nitride layer, formed over the trench sidewalls andover the protective material 509 at the bottom of the trenches 84 a, 84b. FIG. 6J corresponds to FIG. 5K, described above, and illustratesetching the insulating layer 517 and the protective material 509 fromthe bottoms of the trenches 84 a, 84 b to expose the source regions 508of the second semiconductor channel portion 503, while portions of theinsulating layer 517 and the protective material 509 remain over thesidewalls of the trenches 84 a, 84 b.

FIG. 6K corresponds to FIG. 5L, described above, and shows a conductivesource line 102 (e.g., a metal or metal nitride line, such as W, Ti,TiN, etc.) formed over the insulating layer 517 and the protectivematerial 509 in each of the trenches 84 a, 84 b such that the sourceline 120 contacts a source region 508 of the second semiconductorchannel portion 503. The source line 120 is electrically isolated fromthe control gate electrodes 3 by the insulating layer 517 and protectivematerial 509 along the sidewalls of the trench 84 a, 84 b.

Although the foregoing refers to particular preferred embodiments, itwill be understood that the invention is not so limited. It will occurto those of ordinary skill in the art that various modifications may bemade to the disclosed embodiments and that such modifications areintended to be within the scope of the invention. All of thepublications, patent applications and patents cited herein areincorporated herein by reference in their entirety.

What is claimed is:
 1. A method of fabricating a memory device,comprising: forming a first stack of alternating layers of a firstmaterial and a second material different from the first material over asubstrate; removing a portion of the first stack to form a first trench;filling the trench with a sacrificial material; forming a second stackof alternating layers of the first material and the second material overthe first stack and the sacrificial material; removing a portion of thesecond stack to the sacrificial material to form a second trench; andremoving the sacrificial material to form a continuous trench throughthe first stack and the second stack.
 2. The method of claim 1, whereinremoving a portion of the first stack to form the first trench comprisesetching the alternating layers of the first material and the secondmaterial to form a pair of first trench sidewalls in the first stackextending from a first end proximate to the substrate to a second enddistal to the substrate, and a distance between the first trenchsidewalls defines a width of the first trench, and wherein removing aportion of the second stack to form the second trench comprises etchingthe alternating layers of the first material and the second material tothe sacrificial material to form a pair of second trench sidewalls inthe second stack extending from a first end proximate to the second endof the first trench to a second end distal to the second end of thefirst trench, and a distance between the second trench sidewalls definesa width of the second trench, and the width of the first trench at thesecond end of the first trench is greater than the width of the secondtrench at the first end of the second trench.
 3. The method of claim 2,further comprising: forming a select line comprising an electricallyconductive material in the continuous trench.
 4. The method of claim 2,wherein the width of the first trench decreases between the second endand the first end of the first trench.
 5. The method of claim 2, whereinthe width of the second trench decreases between the second end and thefirst end of the second trench.
 6. The method of claim 1, wherein thesacrificial material comprises at least one of silicon nitride, siliconoxide, undoped carbon, doped carbon, amorphous carbon, polycrystallinecarbon, and undoped polysilicon.
 7. The method of claim 1, wherein thecontinuous trench forms a first boundary of an active memory cell area.8. The method of claim 7, wherein a second continuous trench forms asecond boundary of the active memory cell area.
 9. The method of claim8, wherein the active memory cell area comprises a plurality of NANDmemory strings extending through the first stack and the second stack,wherein each NAND memory string comprises: a semiconductor channel whichcomprises at least a first potion that extends substantiallyperpendicular to a major surface of the substrate; and at least onememory film located adjacent to the semiconductor channel.
 10. Themethod of claim 9, wherein the at least one memory film comprises ablocking dielectric, a charge trapping layer or floating gate, and atunnel dielectric, and the memory film is located between thesemiconductor channel and at least one of the first material and thesecond material in the first and second stacks.
 11. The method of claim9, wherein the memory device further comprises a second semiconductorchannel portion which extends substantially parallel to the majorsurface of the substrate above or in the substrate.
 12. The method ofclaim 11, wherein the second semiconductor channel portion iselectrically connected to the at least one semiconductor channel in theplurality of NAND strings, and a second memory film portion extendssubstantially parallel to the major surface of the substrate and islocated between the second semiconductor channel portion and a bottomsurface of the first and second continuous trenches, the method furthercomprising: etching through the bottom surface of the continuoustrenches to expose the second memory film portion; forming a sacrificialmaterial over the sidewalls of the continuous trenches; and etching aportion of the sacrificial material and the second memory film portionat the bottom of the continuous trenches to expose the secondsemiconductor channel portion.
 13. The method of claim 12, furthercomprising: performing ion implantation through the continuous trenchesto provide doped source regions in the second semiconductor channelportion.
 14. The method of claim 13, further comprising: removing aremaining portion of the sacrificial material to expose the sidewalls ofthe continuous trenches.
 15. The method of claim 11, wherein thecontinuous trenches extend to the second semiconductor channel portion,the method further comprising: filling a bottom portion of thecontinuous trenches with a protective material; etching the stackthrough the continuous trenches to remove the alternating layers of thesecond material from at least the active memory cell area and formrecesses between the alternating layers of the first material; formingan electrically conductive material in the continuous trenches andwithin the recesses to form control gate electrodes adjacent to thememory film in each of the NAND strings; and removing the electricallyconductive material from the continuous trenches.
 16. The method ofclaim 15, wherein the alternating layers of the second material comprisea sacrificial material, the alternating layers of the first materialcomprise an insulating material, and the electrically conductivematerial comprises at least one of a metal and a metal nitride.
 17. Themethod of claim 16, wherein the second material comprises at least oneof polysilicon and silicon nitride, the first material comprises anoxide and the electrically conductive material comprises at least one oftungsten, tungsten nitride, titanium and titanium nitride.
 18. Themethod of claim 15, wherein the alternating layers of the secondmaterial are not completely removed outside of the active memory cellarea.
 19. The method of claim 18, wherein the control gate electrodescomprise at least one of a metal, metal silicide or a metal nitrideinside the memory cell area and polysilicon and the at least one of themetal, metal silicide or the metal nitride outside of the memory cellarea.
 20. The method of claim 15, further comprising: depositing aninsulating material over the sidewalls of the continuous trenches andthe protective material at the bottom of the trenches.
 21. The method ofclaim 20, further comprising: etching through the insulating materialand the protective material to expose source regions of the secondsemiconductor channel portion while leaving the insulating material onthe trench sidewalls.
 22. The method of claim 21, wherein the insulatingmaterial and the protective material comprise an oxide material.
 23. Themethod of claim 21, further comprising: filling at least one of thecontinuous trenches with an electrically conductive source line suchthat the source line contacts a source region in the secondsemiconductor channel portion and the insulating material is positionedbetween the source line and the control gate electrodes along a sidewallof the continuous trench.
 24. The method of claim 23, wherein the sourceline comprises at least one of titanium, titanium nitride, tungsten andtungsten nitride.
 25. The method of claim 23, wherein each of the NANDmemory strings comprises a source select transistor that comprises athird semiconductor channel portion that is electrically connected at afirst end to the semiconductor channel portion of the NAND string thatextends substantially perpendicular to a major surface of the substrateand is electrically connected at a second end to the source line via thesource region and the second semiconductor portion that extendssubstantially parallel to the major surface of the substrate, and thesource select transistor further comprises a select gate electrode and agate insulating layer that is located between the select gate electrodeand the third semiconductor channel portion.
 26. The method of claim 25,wherein each of the NAND memory strings further comprises a drain selectgate electrode on a top side of the memory device.
 27. The method ofclaim 4, wherein the width of the first trench at the second end of thefirst trench is configured to compensate for misalignment of the firstend of the second trench during a high aspect ratio trench etch.
 28. Themethod of claim 1, wherein the sacrificial material comprises adielectric material.